Small arms have a ceiling of around 10,000 ft, so aircraft flying at 5,000 ft would definitely be vulnerable. And 300mph is fairly slow compared to a small arms

Small arms have a ceiling of around 10,000 ft, so aircraft flying at 5,000 ft would definitely be vulnerable. And 300mph is fairly slow compared to a small arms projectile traveling maybe 1700mph. Even today, fighter jets will often try to stay above 10,000 ft, roughly where small arms and flak peter out. I could easily imagine WWII rifles having some success damaging helicopters, which were (and compared to modern fighters still are) low and slow.